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Old 19th June 2012, 10:11 AM   #21
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If currentbalance is OK you should either remove or short the cap, off course another thing if you later wish to balance currents electronically with a current mirror.

The cap serves no purpose as I see it, as this is a balanced design and NO LTP. Shorted no local feedback, open local feedback.

Last edited by revintage; 19th June 2012 at 10:14 AM.
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Old 19th June 2012, 10:19 AM   #22
piano3 is offline piano3  United Kingdom
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I initially used 20uF in a similar circuit with low mu tubes and had a bit of a problem with record warps so you might start to see bass extension with your circuit at not much over 100uf assuming mu of 16 for 5687 and 100H(?) for the OPT primary.

My circuit had tails about 50 Mohms long

Last edited by piano3; 19th June 2012 at 10:31 AM.
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Old 19th June 2012, 08:59 PM   #23
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Marginal bass-boost with 10u but it is there. It peaks at 70Hz by 0,2dB with some guesstimates of the inductance values.

Shorting the cap gives the best low end extension and is -1dB at ca 15Hz. The other to are -3dB at the same frequency. But as I say the inductances are guesswork with the primary at 100H.

So removing the cap is no option. Either substitute with a big enough or short it.

I anyway get indications of a very good design(apart from the cap).
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Old 20th June 2012, 06:10 AM   #24
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The cap will be increased. The whole cathode arrangement was a lash up to see how things would sound. Typically I have used a 1000uf back to back with 1000uf on all my other designs and this typically works well and is cheap (64V caps).

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Old 20th June 2012, 06:24 AM   #25
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Short it=infinite cap. But from 470u it will not upset frequency-response.

Once again this is not a LTP. If you go for CCS or current mirror add a cap to each cathode to ground and no cap should between cathodes necessary. About the latter it might be a sonic benefit using your previous back to back arrangement, you decide.

Last edited by revintage; 20th June 2012 at 06:38 AM.
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Old 20th June 2012, 06:33 AM   #26
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nothing wrong with LTP there
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Old 20th June 2012, 06:36 AM   #27
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This isnīt a LTP! Itīs a balanced output stage using a smart input transformer arrangement with a twist. LTP is one input shorted and signal to the other.

Last edited by revintage; 20th June 2012 at 06:44 AM.
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Old 20th June 2012, 06:41 AM   #28
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yes , you can use both

but I'm saying that is nothing wrong if one is using LTP stage after xformer

especially this xformer
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Old 20th June 2012, 06:49 AM   #29
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Can use is one thing, Shoogs design is still balanced not LTP.
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Old 20th June 2012, 06:52 AM   #30
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sorry , current one , posted schmtc really isn't ;

but I commented basing on Shoog's plan to include current mirror in cathodes

then , with AC coupling between cathodes , that is enough LTP for me

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